The present invention relates to an apparatus for heating a fuel mixture used in an internal combustion engine, through which the vaporization of the fuel is promoted, thereby improving the starting performance and driving performance of the engine.
Heretofore it has been proposed that the fuel mixture supplied to the internal combustion engine be heated to promote the vaporization of the fuel and to improve the ignition and combustion of the fuel mixture in the engine, whereby the starting performance of the engine is improved and the generation of uncombusted hydrocarbons and carbon monoxide is reduced, thus improving the fuel consumption ratio. Various apparatuses have been proposed for heating the fuel mixture supplied to the engine. A honeycomb-shaped ceramic having a positive temperature coefficient for electrical resistance (hereinafter referred to as "PTCR") was provided between a carburetor and an intake manifold (U.S. Pat. No. 4,108,125); a cylindrically shaped ceramic, coaxially provided with a cylindrical ceramic on its outer surface, the outer ceramic having a PTCR and electrode layers on its inner and outer surfaces, and being provided in an intake manifold; and a heating apparatus in which a radiating plate having a large number of needle projections provided on a ceramic plate having a PTCR, was fitted in an intake manifold just below a carburetor.
However, a honeycomb-shaped ceramic having a PTCR has a low resistance against mechanical or thermal shock and the ceramic having a PTCR is in direct contact with the fuel mixture, resulting in degradation of the ceramic by fuel compositions and humidity contained in the fuel mixture, or gas blown back from the engine, resulting in a reduction of the breakdown voltage and corrosion of the electrode. Although an electric insulating coating is applied on the ceramic to prevent these defects, the thermal conductivity is poor and this type of coating readily swells and peels off. Moreover, when barium titanate comprises part of the PTCR ceramic, and the ceramic is used for a long period of time an air-tight chamber, the breakdown voltage may be reduced. Therefore, such heating apparatuses cannot be used for a long period of time.
The fitting of a PTCR ceramic in a ceramic cylinder results in defects such as the generation of thermal stresses resulting from differences between thermal expansion coefficients of the different ceramics. Additionally, the dimensions of the ceramics to be fitted together must be precise to result in tight fits therebetween and some type of mechanical working is required on an inner wall of an intake and manifold since the heating apparatus is provided therein. Furthermore, when the PTCR heating apparatus is provided with a radiating plate and fitted to the intake manifold wall, the intake manifold wall is also provided with a fluid passage for a cooling fluid, such as water, or a fluid passage for exhaust gas flow causing heating. Therefore, the heating apparatus is limited in position, the design can become complex, resulting in high manufacturing costs and a correspondingly low commercial value.